The present invention relates to monitoring the condition of a substance using impedance spectroscopy to indicate in real or near real time, i.e., while the substance is being used, the physio-chemical condition of a substance based on a correlation of measurements from electrical signals, using a statistical technique, to previously determined values.
The use of impedance spectroscopy techniques to monitor fluid conditions is previously known. For example, U.S. Pat. Nos. 6,433,560, 6,380,746, 6,377,052, and 6,278,281, along with U.S. Published Application 2003/0141882 of Zou et al., all assigned to the assignee of the present invention, all teach different configurations of electrodes for measuring current and then computing values for impedance. Further, the '281 patent teaches comparing the difference in currents at two frequencies with known bulk and interfacial impedance measurements to determine fluid conditions. Similarly, the '052 and '746 patents teach comparing the difference in impedance values calculated from exciting electrodes at two frequencies to determine fluid conditions.
The value of using impedance spectroscopy to monitor fluid conditions lies in the fact that it is desirable to be able to determine when a fluid, for example, an engine lubricant, has degraded to the point where it has either exhausted or come close to exhausting its useful life. Similarly, it is desirable to know how many hours of useful life remain with respect to a fluid sample. For an application such as monitoring the condition of an engine lubricant, it is desirable to be able to monitor the fluid condition while the engine is operating, as opposed to performing tests in a laboratory.
Lubricating fluids comprise three basic components: (1) base stock, (2) additives, and (3) contaminants. These components are known to influence the bulk and interfacial properties of the lubricant. Lubricating fluids possess interfacial properties, such as wear protection and corrosion protection, which are present at the interface between the fluid and the metal it protects. Different portions of the impedance spectrum correspond to bulk and interfacial properties. Previous applications of impedance spectroscopy have measured bulk properties and interfacial properties separately, but the capability of measuring the two together has not been previously appreciated. Accordingly, it would be desirable if bulk and interfacial properties could be measured together in order to give a more complete picture of the engine lubricant.
Existing methods of monitoring fluid conditions using impedance spectroscopy fail to contemporaneously measure a plurality of fluid properties. Rather, prior art methods, including those disclosed by the above-mentioned patents and publication, calculate impedance in different ways in order to determine a value for one fluid property. It can be shown that up to 90% of available information is not utilized when single parameter measurement techniques are employed. Thus, there is a need for systems and methods capable of implementing a multiple parameter function or functions to make quantitative measures of a broad range of fluid condition metrics.